Light olefins such as ethylene, propylene and butadiene are important basic raw materials in the petrochemical industry and are mainly produced by the cracking furnace steam cracking process at present. Statistics shows that about 99% of ethylene, more than 50% of propylene and more than 90% of butadiene in the world are produced by the above process.
A process unit based on the cracking furnace steam cracking process and the downstream cryogenic separation process as essential technologies is called an ethylene plant. A cracking furnace comprising a convection section and a radiant section is the key apparatus of an ethylene plant, wherein feed stocks and diluted steam are first separately heated in the convection section, and then mixed, vaporized and heated to an initial cracking temperature (i.e., a crossover temperature) before entering into the radiant section for cracking reaction. Generally, the radiant section of an industrial cracking furnace is provided with a plurality of furnace tubes of the same composition and configuration. A feed stock is fed into the furnace tubes, the outer ails of which are heated by heat released from liquid or gas fuel combustion. The heat is then transferred to the feed stock in the furnace tubes through the outer walls.
As is known to all, cracking is a process whereby carbon-carbon bonds in saturated petroleum hydrocarbons are broken down or dehydrogenated under high temperature into olefins and other products. The object of cracking is to produce ethylene and propylene, with side products such as butene, butadiene and other olefins and pyrolysis gasoline, diesel, fuel oil, etc.
Synthesized rubber and resin with butadiene as the monomer have been so rapidly developed in recent years that the prices of butadiene products are increasingly high and butadiene products have become important profit sources of ethylene plants. Different feed stocks lead to different butadiene yields. Gas feed stocks (lighter than C5 hydrocarbons) usually bring about a comparatively low butadiene yield, which for example is only about 4% in n-butane cracking products, while liquid feed stocks (such as naphtha, hydrogenated cracking residue, etc.) would result in a comparatively high yield of butadiene, which for example can be as high as 7% in hydrogenated cracking residue cracking products. Some of the olefins generally considered as incapable of serving as feed stocks may produce quiet high yield of butadiene, which for example can surprisingly reach as high as 18% in the catalytic cracking products of cis-butene. Therefore, the yield of butadiene can be expected to increase with addition of olefins such as cis-butene in the catalytic cracking furnace for cracking reaction.
The cracking portion of an ethylene plant usually comprises a plurality of liquid cracking furnaces and one gas cracking furnace. Feed stocks for the gas furnace are generally ethane, propane, C4 alkanes, etc., which are fed into the furnace in the gas phase and do not have to be vaporized in the convection section of the cracking furnace, so that the cracking furnace can be simply designed. Liquid cracking furnaces, on the other hand adopt naphtha, diesel, hydrogenated cracking residue, etc. as raw materials, which are fed into the furnaces in the liquid phase and thus need to be vaporized in the convection section of the cracking furnaces usually with complex structures.
Generally speaking, the convection section of a cracking furnace mainly functions in two aspects, for one thing, to preheat, vaporize and overheat the feed stock to the initial cracking temperature (the crossover temperature) and for another to recover the exhaust heat in the flue gas, so that the thermal efficiency of the furnace can be improved. Therefore, in view of different process requirements, the convection section normally employs different heat exchange arrangements, and mainly comprises a material preheating segment, a boiler feedwater preheating segment, a diluted steam overheating segment, a high pressure steam overheating segment and a hybrid heating segment. The convection section of the cracking furnace is continuously developed as technology develops. On the one hand, the number of the convection sections is increasingly larger. For example, in accordance with the amount of flue gas heat, the material preheating segment can be divided into the upper material preheating segment, mid-material preheating segment and lower material preheating segment. On the other hand, the feeding manner of diluted steam can be as diversified as comprising the one-off feeding manner and the secondary feeding manner based on different raw materials. These different feeding manners are adopted to prevent the raw material from being coked in the convection section. When liquid raw materials such as naphtha, diesel, hydrogenated cracking residue, etc. are used, a vaporization process exists in the convection section in the heating process, wherein if the raw material contains olefins, then at the beginning of the vaporization process, a high content of olefins in the gas phase would easily cause formation of coke, and when the vaporization process is to be ended, hydrocarbon components in the liquid phase would be so heavy that coke is also easily formed. In case severe coke is formed in the convection section of the cracking furnace, not only the heat transfer process would be seriously affected, but the pressure drop therein would also be rapidly increased, which would reduce the yield in the cracking furnace. When coke is accumulated to a certain limit, the cracking furnace will have to be shut down for mechanical decoking. In the prior art, the content of olefins in liquid feed stocks for cracking generally cannot be higher than 2 wt %. Once the content of olefins is too high, subsequent problems would be brought about, such as the formation of coke in the convection section of the cracking furnace and great decrease in the operation cycle of the cracking furnace, whereby causing maintenance shutdown of the cracking furnace.
Some embodiments are disclosed in the prior art for feeding various kinds of feed stocks into the convection section of a cracking furnace. For example, CN 1077978 A discloses a process for preparing ethylene by petroleum hydrocarbon steam cracking in the convection section. The process adopts the twice injection mode, i.e., primary steam injection at three points and secondary steam injection at one point, so that the cracking furnace is adapted to not only light materials but also heavy materials. Moreover, pipe lines are unnecessary to be replaced in the switch between raw materials. Nevertheless, the above patent application merely relates to improvement of steam injection modes, which does not influence the cracking yield or product quality in the whole cracking process.
CN 1501898 A discloses a process for cracking light feeding material in a cracking furnace for cracking heavy feeding material, comprising feeding part of the light material through an inlet of the convection section of the cracking furnace and feeding the rest light material into the convection section together with diluted gas. This process solves the problem of feeding light material into the cracking furnace when heavy material is replaced by light material, wherein an excessive pressure drop can be prevented when the light material passes through a preheating section.
US 2009/0178956 A1 discloses a process of reducing formation of coke of liquid feed stock in the convection section, wherein the partial pressure of the liquid material is reduced by feeding a gas phase when the liquid material is being preheated so as to improve the vaporization rate of the resulting mixture of the liquid material and the dilution steam and delay the formation of coke precursors of the liquid material, thus reducing or even eliminating formation of coke of the liquid material in the convection section.
Currently, steam cracking processes focus on how to enable the cracking furnace to be suitable for a variety of materials, for example from light to heavy materials, or on how to slow down or eliminate the formation of coke when heavy material is being used. In the prior art, there is limited disclosure relating to the process of feeding olefins (a monoolefin-containing stream) as part of the feed stock into the cracking furnace for steam cracking, not to mention eliminating formation of coke when olefins are injected into the cracking furnace as part of the feed stock.
Usually, the feed stock is preheated in the convection section of a cracking furnace before entering into the radiant section for cracking reaction, wherein the feed stock absorbs heat to so high a temperature that cracking reaction is generated to produce target products such as ethylene, propylene, butadiene, etc. At the outlet of the radiant section of the furnace, the cracked gases can react for a second time under high temperature to produce side products. Therefore, the high-temperature cracked gases need to be rapidly cooled at the outlet of the radiant section of the furnace to prevent too many secondary reactions to affect yield of the target products. For cooling of the cracked gases, both the direct quenching method and the indirect quenching method can be adopted, wherein the direct quenching method means directly contacting cryogens with the cracked gases to cool the gases rapidly, while the indirect quenching method means indirectly contacting cryogens with the cracked gases through a wall to cool the gases rapidly. The indirect quenching method is usually adopted in order to recover the heat of high-temperature cracked gases so as to improve the thermal efficiency of the cracking furnace and reduce the costs of the products, wherein a quench heat exchanger is used, i.e., a transfer line exchanger (TLE) is used for cooling the cracked gases rapidly and recover the heat to produce steam.